The use of multilayer reflective films comprising alternating layers of two or more polymers to reflect light is known and is described, for example, in U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.), U.S. Pat. No. 5,103,337 (Schrenk et al.), WO 96/19347, and WO 95/17303. The reflection and transmission spectra of a particular multilayer film depends primarily on the optical thickness of the individual layers, which is defined as the product of the actual thickness of a layer times its refractive index. Accordingly, films can be designed to reflect infrared, visible or ultraviolet wavelengths .lambda..sub.M of light by choice of the appropriate optical thickness of the layers in accordance with the following formula: EQU .lambda..sub.M =(2/M)*D.sub.r (Formula I)
wherein M is an integer representing the particular order of the reflected light and D.sub.r is the optical thickness of an optical repeating unit (also called a multilayer stack) comprising two or more polymeric layers. Accordingly, D.sub.r is the sum of the optical thicknesses of the individual polymer layers that make up the optical repeating unit. D.sub.r is always one half lambda in thickness, where lambda is the wavelength of the first order reflection peak. By varying the optical thickness of an optical repeating unit along the thickness of the multilayer film, a multilayer film can be designed that reflects light over a broad band of wavelengths. This band is commonly referred to as the reflection band or stop band.
It is desirable for a reflection band to have a sharp spectral edge at the long wavelength (red) and/or short wavelength (blue) side. However, the reflective films known to the art that contain an optical repeating unit of varying optical thickness typically have moderately sloped bandedges which cause reflections outside of the desired wavelengths of interest. For example, if a reflective film is designed to reflect infrared light while being transparent over the visible spectrum, a sloped edge on the blue side of the reflection band may encroach into the visible region of the spectrum, thereby resulting in unwanted coloring of the infrared reflective film body. Such coloring can be avoided by designing the infrared film such that the infrared reflection band is moved further into the infrared region, but this results in substantial transmission of infrared light near the visible region of the spectrum.
In other situations, it may be desirable to design a reflective film or other optical body that reflects light over a selected range in the visible region of the spectrum, e.g., a reflective film that reflects only green light. In such a case, it may be desirable to have sharp edges at both the red and blue sides of the reflection band.
Many prior art reflective films comprising multilayer stacks also show a number of small reflection peaks near the reflection band. This so-called "ringing" also may introduce unwanted reflections. It has been suggested in the art that, for multilayer films that consist of an optical repeating unit of constant optical thickness, such ringing might be suppressed by adding a number of optical repeating units having an optical thickness of half that of the other optical repeating units responsible for the reflection band. However, while this approach may eliminate ringing, it does not improve bandedge sharpness and, in fact, may worsen it. Furthermore, this approach requires the presence of strippable skins on multilayer extruded films, since it permits only thin layers of specific optical thickness on the surface.
There is thus a need in the art for a reflective film, and a method of making the same, that exhibits a sharp bandedge on one or both sides of the main reflection band, and that avoids the presence of ringing and other undesirable reflections. These and other needs are met by the present invention, as hereinafter described.